As mobile information devices become more prevalent, there is an increasing demand for a longer continuous operable time and a longer charging battery life of those devices. Especially, there is a strong demand for reducing the power consumption of an RF (Radio Frequency) analog circuit which is the basis of a wireless system.
FIG. 8 shows an arrangement of the RF analog circuit in a general wireless system. FIG. 9 shows a result obtained by measuring a distribution of the power consumption in the RF analog circuit.
The RF analog circuit generally includes a transmitter 51, a power amplifier 52, a synthesizer 53, an LNA (Low Noise Amplifier) 55, and a receiver 54. A transmitting signal having been outputted from the transmitter 51 is amplified by the power amplifier 52, and then the amplified transmitting signal is transmitted. Meanwhile, a receiving signal is amplified by the LNA 55, and then is subjected to a receiving processing (frequency conversion, demodulation, etc.) by the receiver 54. The frequency synthesizer 53 provides a local signal, necessary for processings, to the transmitter 51, the receiver 54, and the LNA 55.
According to the distribution of the power consumption shown in FIG. 9, the total of the power consumptions of the transmitter 51, the LNA 55, the receiver 54, and the synthesizer 53 is about 800 mW, while the power consumption of the power amplifier 52 is 1,000 mW. That is, the power consumption of the power amplifier 52 accounts for a large part of the power consumption of the RF analog circuit. Therefore, it is evident that reducing the power consumption of the power amplifier 52 is effective to reduce the power consumption of the RF analog circuit. In addition, the power amplifier 52 has a simple circuit configuration, so that the power amplifier 52 can operate even if a power supply voltage is decreased. On this account, it is easy to save power. Moreover, the power consumption of the power amplifier 52 can be adjusted in accordance with a transmitting power (target value). For these reasons, controlling the power consumption of the power amplifier 52 is the best way to reduce the power consumption of the RF analog circuit.
Many approaches to reduce the power consumption of the RF analog circuit in a conventional wireless system have been made mainly by adjusting the power supply voltage and a bias voltage of the power amplifier 52. A conventional technology of adjusting the power supply voltage of an RF power amplifier is disclosed in, for example, U.S. Pat. No. 6,148,220 (published on Nov. 14, 2000). The following explains such conventional technology.
FIG. 10 shows a simplified arrangement of the conventional technology. As shown in FIG. 10, the arrangement of the conventional technology includes an RF transmitter 101, a power amplifier 102, a power measuring circuit 103, a controller 104, and a variable voltage regulator 105 (power supply voltage adjusting circuit). In this arrangement, a high frequency transmitting signal supplied from the RF transmitter 101 is amplified by the power amplifier 102, and then the power of the high frequency transmitting signal is detected by the power measuring circuit 103. The controller 104 provides an instruction (supplies an adjusting signal) to the variable voltage regulator 105 in accordance with the detected power of the transmitting signal so that the variable voltage regulator 105 controls the power supply voltage. In addition, the controller 104 also provides an instruction (supplies an adjusting signal) to a bias adjusting circuit 106 of the power amplifier 102 in accordance with the detected power of the transmitting signal so that the bias adjusting circuit 106 adjusts the bias voltage. Thus, the variable voltage regulator 105 adjusts the power supply voltage of the power amplifier 102 in accordance with the power of the transmitting signal. Moreover, the power amplifier 102 changes the bias voltage in accordance with the power of the transmitting signal.
In the above arrangement of the conventional technology, the power supply voltage and the bias voltage of the power amplifier 102 are adjusted to improve characteristics. However, the conventional technology does not refer to a quantitative adjustment range which indicates to what extent the power supply voltage and the bias voltage can be adjusted without deteriorating a signal quality. Therefore, even in the case in which signal distortion occurs when the power supply voltage of the power amplifier 102 is reduced, the deterioration in signal quality cannot be improved.
Because variations in characteristics, such as a threshold voltage and the like, of a transistor are increased due to a deeper submicron semiconductor process, variations in characteristics of the circuit also become significant. On this account, a method for adjusting the power supply voltage to obtain an optimal power supply voltage for each device becomes necessary.
Therefore, in the case of employing the conventional technology, there is no other choice but to (i) take an excessive margin with respect to a lower limit of the power supply voltage to secure a high signal quality or (ii) adjust the power supply voltage and the bias voltage while ignoring the signal quality.